Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Echo”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Fast magnetospheric echoes of energetic electron beams

Electron beam experiments using rocket-borne instrumentation confirmed earlier observations of fast magnetospheric echoes of artificially injected energetic electrons. A total of 234 echoes were observed in a pitch angle range from 9 to 110 deg at energies of 1.87 and 3.90 keV. Of these, 102 echoes could unambiguously be identified with known accelerator operations at 2, 4 or 8 keV energy and highest current levels resulting in the determination of transit times of typically 300 to 400 ms. In most cases, when echoes were present in both energy channels, the higher energy electrons led the lower energy ones by 50 to 70 ms. Adiabatic theory applied to these observations yields a reflection height of 3000 to 4000 km. The injection process is discussed as the strong beam-plasma interaction that occurred near the electron accelerator appears to be instrumental in generating the source of heated electrons required for successful echo detection.

Wilhelm, K.↗
NASA NTRS1984

Elimination of range-aliased echoes in the VHF radars

Very high frequency radars designed to measure tropospheric wind profiles usually detect scattering to a maximum height of about 20 km. If the antenna elevation angle is 45 degrees or more above the horizon, the maximum range of interest is less than 30 km. A VHF pulsed Doppler radar wind Profiler can, therefore, be operated at high pulse repetition rates. The maximum bandwidth allowed is about 0.5 MHz so a radar with uncoded pulses can operate with a duty cycle of 1 to 10%, depending on the desired height resolution. It is possible to operate a tropospheric wind profiler that utilizes all the average power available from the transmitter without the complexity of coded pulses. However, the VHF radar can detect echoes from the mesosphere on occasion and, with high pulse repetition rates, these echoes occur at the same apparent range as the tropospheric echoes. These mesospheric echoes may be stronger than the tropospheric signals. The range-aliased mesospheric echoes can be greatly attenuated or effectively eliminated.

Strauch, R. G.↗
NASA NTRS1985

Fast magnetospheric echoes of energetic electron beams

Electron beam experiments using rocketborne instrumentation have confirmed earlier observations of fast magnetospheric echoes of artificially injected energetic electrons. A total of 234 echoes have been observed in a pitch angle range from 9 to 110 deg at energies of 1.87 and 3.90 keV. Out of this number, 95 echoes could unambiguously be identified with known accelerator operations at 2-, 4-, or 8-keV energy and highest current levels resulting in the determination of transit times of typically 300 to 400 ms. In most cases, when echoes were present in both energy channels, the higher-energy electrons led the lower-energy ones by 50 to 70 ms. Adiabatic theory applied to these observations yields a reflection height of 3000 to 4000 km. An alternative interpretation is briefly examined, and its relative merit in describing the observations is evaluated. The injection process is discussed in some detail as the strong beam-plasma interaction that occurred near the electron accelerator appears to be instrumental in generating the source of heated electrons required for successful echo detection for both processes.

Wilhelm, K.↗
NASA NTRS1992

Diurnal variation of overdense meteor echo duration and ozone

The diurnal variation of the median duration of overdense sporadic radar meteor echoes is examined. The meteors recorded in August, December, and January by the Ondrejov meteor radar during the period 1958-1990 were used for the analysis. A maximum median echo duration 1-3 hours after the time of local sunrise in the meteor region confirms the already known sunrise effect. Minimum echo duration occurring at the time of sunset seems to be the most important point of diurnal variation of the echo duration, when ozone is no longer dissociated by solar UV radiation. The effect of diurnal changes of the echo duration should be considered when the mass distribution of meteor showers is analyzed.

Simek, Milos↗
NASA NTRS1992

Observations of the light echoes from SN 1987A using the Astro-1 Ultraviolet Imaging Telescope

We present the results from images taken from the region of the light echoes around SN 1987A, as acquired on the first flight of the Astro-1 Ultraviolet Imaging Telescope (UIT). They indicate a weighted-average UV echo surface brightness of about 3 x 10 exp -18 ergs/s sq cm A sq arcsec. This is consistent with earlier results obtained by the IUE when scaled by the optical surface brightness of the two different echoes observed. These results indicate that the UV flux emitted by shock from core collapse penetrating the stellar surface cannot be as strong as that predicted by a large class of theoretical models (cited herein), or that previous results on the optical scattering of echoing dust do not apply to these clouds. Prospects for a more accurate measurements once the echoes have propagated to other regions and a background measurements can be obtained with UIT are discussed. They indicate that a more accurate determination of the above results is probable with another epoch of UIT observations.

Crotts, Arlin P. S.↗
NASA NTRS1994

Light echo detection of circumstellar disks around flaring stars

Light echoes can be used to detect and characterize disks around flaring stars. Such disks are thought to be a hallmark of planet formation but are very difficult to detect by ordinary means. Dwarf emission-line M stars experience flares with luminosities comparable to their quiescent photospheres on time scales of minutes, less than the light travel time across a disk many astronomical units in extent; they are thus ideal candidates for such a search. Bromley (1992) calculated that the detection of Jupiter-sized companions using light echoes requires photometric accuracies better than 1 part in 10(exp 6). However, a disk consisting of grains or small bodies will scatter a much larger fraction of the light than a planet of similar mass. I estimate the light echo amplitutdes from plausible geometries of circumstellar material and present simulation light curves. The light echo amplitudes are typically 1% of the flare and I conclude that such events will be detected best in cases where the flare is eclipsed by the star. An examination of the time scales associated with internal processes in a protoplanetary disks around dM stars indicates that any primordial disks may become undetectable in 10(exp 4) years and will have completely disappeared by 10(exp 8) years, the estimated age of dMe stars in the solar neighborhood. However, searches for light echoes might constrain the amount of material continuing to fall into these young stellar systems in the form of comet-like objects.

Gaidos, Eric J.↗
NASA NTRS2004

Modeling of Field-Aligned Guided Echoes in the Plasmasphere

The conditions under which high frequency (f>>f(sub uh)) long-range extraordinary-mode discrete field-aligned echoes observed by the Radio Plasma Imager (RPI) on board the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite in the plasmasphere are investigated by ray tracing modeling. Field-aligned discrete echoes are most commonly observed by RPI in the plasmasphere although they are also observed over the polar cap region. The plasmasphere field-aligned echoes appearing as multiple echo traces at different virtual ranges are attributed to signals reflected successively between conjugate hemispheres that propagate along or nearly along closed geomagnetic field lines. The ray tracing simulations show that field-aligned ducts with as little as 1% density perturbations (depletions) and less than 10 wavelengths wide can guide nearly field-aligned propagating high frequency X mode waves. Effective guidance of wave at a given frequency and wave normal angle (Psi) depends on the cross-field density scale of the duct, such that ducts with stronger density depletions need to be wider in order to maintain the same gradient of refractive index across the magnetic field. While signal guidance by field aligned density gradient without ducting is possible only over the polar region, conjugate field-aligned echoes that have traversed through the equatorial region are most likely guided by ducting.

Fung, Shing F.↗
NASA NTRS2007

Solar Sail Model Validation from Echo Trajectories

The NASA In-Space Propulsion program has been engaged in a project to increase the technology readiness of solar sails. Recently, these efforts came to fruition in the form of several software tools to model solar sail guidance, navigation and control. Furthermore, solar sails are one of five technologies competing for the New Millennium Program Space Technology 9 flight demonstration mission. The historic Echo 1 and Echo 2 balloons were comprised of aluminized Mylar, which is the near-term material of choice for solar sails. Both spacecraft, but particularly Echo 2, were in low Earth orbits with characteristics similar to the proposed Space Technology 9 orbit. Therefore, the Echo balloons are excellent test cases for solar sail model validation. We present the results of studies of Echo trajectories that validate solar sail models of optics, solar radiation pressure, shape and low-thrust orbital dynamics.

Heaton, Andrew F.↗
NASA NTRS2016

Stellar Echo Imaging of Exoplanets

All stars exhibit intensity fluctuations over several timescales, from nanoseconds to years. These intensity fluctuations echo off bodies and structures in the star system. We posit that it is possible to take advantage of these echoes to detect, and possibly image, Earth-scale exoplanets. Unlike direct imaging techniques, temporal measurements do not require fringe tracking, maintaining an optically-perfect baseline, or utilizing ultra-contrast coronagraphs. Unlike transit or radial velocity techniques, stellar echo detection is not constrained to any specific orbital inclination. Current results suggest that existing and emerging technology can already enable stellar echo techniques at flare stars, such as Proxima Centauri, including detection, spectroscopic interrogation, and possibly even continent-level imaging of exoplanets in a variety of orbits. Detection of Earth-like planets around Sun-like stars appears to be extremely challenging, but cannot be fully quantified without additional data on micro- and millisecond-scale intensity fluctuations of the Sun. We consider survey missions in the mold of Kepler and place preliminary constraints on the feasibility of producing 3D tomographic maps of other structures in star systems, such as accretion disks. In this report we discuss the theory, limitations, models, and future opportunities for stellar echo imaging.

stellar↗
NASA NTRS1976

Spearhead echo and downburst near the approach end of a John F. Kennedy Airport runway, New York City

Radar echoes of a storm at John F. Kennedy International Airport are examined. Results regarding the phenomena presented suggest the existence of downburst cells. These cells are characterized by spearhead echoes. About 2% of the echoes in the New York area were spearhead echoes. The detection and identification of downburst cells, their potential hazard to approaching and landing aircraft, and communication of this information to the pilots of those aircraft are discussed.

Fujita, T. T.↗
NASA NTRS1983

Fast magnetospheric echoes of artificially injected electrons observed above a bright auroral arc

Rocket-borne electron beam experiments confirmed earlier observations of fast magnetospheric echoes of artificially injected energetic electrons. A total of 234 echoes were observed at pitch angles from 9 to 10 deg at energies of 1.87 and 3.90 keV. Of these, 102 echoes are unambiguously related to preceding accelerator operations at 2, 4 or 8 keV energy and highest current levels resulting in the determination of transit times of typically 300 to 400 msec. When echoes are present in both energy channels, higher energy electrons lead lower energy ones by 50 to 70 msec. Adiabatic theory applied to the observations yields a reflection height of 3000 to 4000 km.

Wilhelm, K.↗
NASA NTRS1985

A theoretical model of artificial spread F echoes

A theoretical model is developed for artificial spread F echoes elicited by irradiating the ionospheric F region with signals from ground-based HF transmitters. Account is taken of the irregularity polarizations, scale length and the magnetic dip angle of the echo. Ray tracing equations are defined for wave propagation in a horizontally stratified ionosphere which has been bathed with HF signals and therefore contains wavelike structures. Irregularities polarized within the meridian plane are found to cause the spread F echoes, while perpendicularly polarized irregularities do not. A magnetic dip angle of 5 deg must be exceeded for the spread F to become strong. The irregularities need scale lengths exceeding 100 m. Modes of operation are identified for inducing the required spread F echoes using ground-based radiotelescopes.

Kuo, S. P.↗
NASA NTRS1985

Objectives and design of the Echo 6 electron beam experiment - Large ionospheric perturbations and energetic particle patterns

The data from the March 30, 1983 launching of Echo 6 are presented. Echo 6 contained duplex accelerators and a system of four throw away detectors to detect and analyze conjugate electron echoes. The model used to calculate the proper trajectory of the rocket and the reason for not detecting echoes are discussed. The components of the plasma diagnostic package which was used to study the electron beam interactions with the ionospheric plasma, and the functions of the two controllable electron guns and the two sets of orthogonal electric probes are described. The signal-mode potentials between each probe and the plasma diagnostic payload are investigated. The signals produced by the gun pulses are analyzed and the interpretation of the electric field probe response is examined.

Winckler, J. R.↗
NASA NTRS1987

Measurement of ice accretion using ultrasonic pulse echo techniques

Many figures are given to illustrate the measurement of ice deposition using ultrasonic pulse echo techniques. The basic concept is to measure the thickness of the ice by relating the pulse echo time to the speed of sound. The measurements are made in an icing research tunnel (IRT), where echo patterns are videotaped during icing exposures under a variety of conditions. Typical echo patterns for different types of ice are illustrated. A table summarizing the icing rates measured in the IRT, along with the presence or absence of surface water is also given.

Hansman, R. John, Jr.↗
NASA NTRS1988

Light echoes - Novae

The sudden brilliance of a nova eruption will be reflected on surrounding dust grains to create a phantom nebula. Previous searches for these light echoes have used relatively short exposures with photograhic detectors. This paper reports on a search around eight recent novae with long exposures using a CCD camera. Despite an increase of sensitivity by over an order of magnitude, no light echoes were detected. It is found that the average grain density must be less than about 10 to the -9th per cu cm for distances from 0.1 pc to 1000 pc from the novae. The light echo around Nova Persei 1901 was caused by reflection off clouds with grain densities of several times 10 to the -9th per cu cm which are at distances between 0.1 pc and 10 pc. Echoes from dust in a circumstellar shell or ejected during a previous eruption will be effectively unobservable.

Schaefer, Bradley E.↗
NASA NTRS1989

A model for depolarized radar echoes from Mars

The depolarized radar echoes from Mars are modeled using a combination of remote-sensing observations. The model reproduces the variations of the total radar cross-sections with longitude observed by Goldstone (1986) along 7 S, yields larger magnitudes of total radar cross-sections along 22 N than those along 7 S, and produces depolarized echo spectra that broadly match those observed by the Arecibo radar in 1980 and 1982. The model indicates that volcanoes and lava plains of the Tharsis-Alba Patera, Elysium, and Amazonia regions have the strongest depolarized echoes from the entire planet. Rock populations for the moon and Mars are estimated assuming depolarized radar echoes result from rocks with radii between 1.3 and three times the wavelength.

Thompson, T. W.↗
NASA NTRS1989

Light echoes and transient luminescence near SN 1987A

The discovery of two new light echoes from sheets of material behind supernova 1987A and present images of the progenitor's circumstellar shell are reported, indicating diffuse echoes from the star's red giant wind. The echo sheets' geometry explains well the behavior of SN 1987's 10-micron flux, but the circumstellar shell appears to be 70 percent larger than the prediction from the analysis of narrow UV emission lines. The sheets' recombination time show them relatively thin and dense. The data also constrain the existence of any fourth star in the Sanduleak -69 deg 202 system and show that the feature reported 8 arcsecs from the supernova is probably not an echo from a thin sheet in SN 1987A's foreground.

Crotts, Arlin P. S.↗
NASA NTRS1990

Echo size and asymmetry - Impact on NEXRAD storm identification

The effects of echo shape and radar viewing angle on detecting small thunderstorms with the NEXRAD storm identification algorithms are examined. The amorphous low-level echo shapes are modeled as ellipses with major axes ranging from 5-15 km and minor axes varying between 2-5 km. The model echoes are then used to create a 'probability of detection' chart that demonstrates the impact of storm asymmetry on cell identification. The algorithm performance on small thunderstorms observed near Huntsville, Alabama and Kennedy Space Center, Florida is examined. A new algorithm based on the analysis of 15 storms observed in Florida, Alabama, and New Mexico is proposed that would identify storms as having lightning if 40 dBZ reflectivity is present at the -10 C level and the echo top exceeds 9 km. This algorithm would have a 100 percent probability of detecting lightning producing storms 4-33 min before the first flash, a 7 percent false alarm rate and a critical success index of 93 percent.

Buechler, Dennis E.↗